At 01:08 AM 1/4/2007, Telegrapher9@aol.com wrote:
>Gene,
>
>does anyone run simulations or calculations on lightning strikes.
Indeed they do. There's a lot of literature out there on
it. Recently, there have been some interesting papers trying to
quantify things other than the "direct hit".. that is, the induced
voltages and currents from "nearby hits" and/or EMP. Interestingly,
one of the reasons for NEC2 being enhanced to NEC3 and NEC4 was for
better modeling of a impulse simulation setup.
However, there's also a fair amount of empirical data. For instance,
researchers have put recorders on long aboveground telephone and
power lines and recorded the transients and accumulated statistics on
what sort of rise times, voltages, etc.
> A common
>direct stike is 100 kA with a risetime of 8 us, correct?
A more common waveform would be rise time of 2 microseconds, fall to
50% in 50 microseconds. Typical average stroke is 20-30kA. 100kA
would be a big-un
>If so, using an
>inductance of 200 nH/ft, the voltage drop along 1 foot of wire is
>2.5 kV. And we can
>calculate the low frequency impedance of a ground system. Let's say it is 1
>ohm. Now we have 100 kV between the tower base and earth ground.
I use 1 uH/meter for wires, and I think 1 ohm is a bit optimistic,
but your example is in the ballpark, and shows you can get BIG
voltages with relatively short runs.
FWIW, above ground power lines have a characteristic impedance of
around 300 ohms.
>Using some ball park numbers like this we should be able to determine the
>currents and potentials at various points in the system. Then
>appropriate series
>Z and shunt Z can be applied to design for survival of equipment.
This is what Ronald Standler's book is all about. He gives the
models, the calculations, etc.
Standler, Protection of Electronic Circuits from Overvoltages,
New York: Wiley-Interscience, 434 pp., May 1989. Republished by
Dover, December 2002
It's on Dover's website, and it's about $26, softcover.
http://store.doverpublications.com/0486425525.html
>I'm thinking of doing it via design rather than blindly by code. I suppose
>that there is nearly 100 years of experience contained in the codes that will
>lead one to success.
If by codes you mean electrical codes, the "success" criteria is
"building not catching fire and people not being killed". Kind of
different from "equipment not being cooked".
>But perhaps a good understanding of the nuances can help
>one to gain several dB of margin by some simple additions.
>
>Can someone steer me to some numbers on lightning? Thanks.
http://home.earthlink.net/~jimlux/lfacts.htm
But, another good book from Dover publishers is "Lightning" by Martin Uman
http://store.doverpublications.com/0486645754.html $11
Very handy.
> Dave WX7G
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